Positron imaging has clearly demonstrated its potential for the study and diagnosis of functional disease. However the future of positron imaging will depend on the development of positron emission tomography (PET) with higher sensitivity and resolution. To achieve this goal, PCR-II, a cylindrically shaped analog coded positron tomograph, has been designed and is in the final stages of construction. The instrument will approach the fundamental limits of sensitivity and resolution of positron detection. Algorithms have been developed for three-dimensional reconstruction of data from PCR-II. Algorithms for Fourier space reconstruction are available as well as algorithms for conventional filtered backprojection. A Monte-Carlo simulation code has been used to assist in this process. On initial completion of PCR-II, the instrument will undergo an intensive program of physical study including phantom imaging. Animal studies will be carried out to determine the performance of the instrument and to demonstrate the biological significance of high resolution, quantitative PET imaging. Biological imaging will lead to ultimate clinical research application. Biological and clinical applications will be carried out with collaboration at MGH and elsewhere. The refinement of PCR-II hardware and software will continue, however, over the three year period. Studies using PCR-I in animal models will continue until PCR-II is fully operational. These collaborative projects will involve a number of other research groups. Following initial study of PCR-II, attention will be given to approaches to simplify construction and reduce electronic complexity.